An elevator comprises an elevator car, lifting machinery, ropes and a counter weight. The elevator car is supported on a sling surrounding the elevator car. The lifting machinery comprises a traction sheave, a machinery brake and an electric motor being connected via a shaft. The electric motor is used to rotate the traction sheave and the machinery brake is used to stop the rotation of the traction sheave. The lifting machinery is situated in a machine room. The lifting machinery moves the car upwards and downwards in a vertically extending elevator shaft. The elevator car is carried through the sling by the ropes, which connect the elevator car over the traction sheave to the counter weight. The sling is further supported with gliding means at guide rails extending in a vertically directed elevator shaft. The gliding means can comprise rolls rolling on the guide rails or gliding shoes gliding on the guide rails when the elevator car is mowing upwards and downwards in the elevator shaft. The guide rails are supported with fastening brackets at the side wall structures of the elevator shaft. The gliding means engaging with the guide rails keep the elevator car in position in the horizontal plane when the elevator car moves upwards and downwards in the elevator shaft. The counter weight is supported in a corresponding way on guide rails supported on the wall structure of the shaft. The elevator car transports people and/or goods between the landings in the building. The elevator shaft can be formed so that the wall structure is formed of solid walls or so that the wall structure is formed of an open steel structure.
The machinery brake is an electromechanical brake that stops the rotation of the traction sheave. The machinery brake comprises a brake disc connected to the shaft connecting the electric motor, the traction sheave and the machinery brake. The brake disc is positioned between a stationary frame and an armature plate. A spring acts against the armature plate, whereby the brake disc is pressed between the armature plate and the stationary frame flange. There are further coils acting on the armature plate in the opposite direction i.e. against the force of the spring. The brake is open when current is supplied to the coils. The magnetic force of the coil moves the armature plate against the force of the spring away from the surface of the brake disc. The spring will immediately press the brake disc between the armature plate and the stationary frame flange when the current supply to the coils is disconnected. Two coils are used for safety reason.
It is advantageous that the electric motor already produces the required torque in the right direction when the machinery brake is beginning to loosen the grip of the brake disc. This will eliminate twitches in the start of the movement of the elevator car when the elevator system is unbalanced. The people in the elevator car will experience a smooth start and a comfortable ride in this way. The direction and the amount of the torque that is required must thus be determined somehow in advance. This is done in prior art solutions by using the weight sensor of the elevator car. The weight sensor measures the load within the elevator car.
The problem in this prior art solution is that the measured values received from the weight sensor are not very precise and reliable.
There is thus a need for a more precise and more reliable method for controlling an elevator. More precise and reliable information of the direction and the amount of the torque needed in each situation, in order to be able to start the ride of the elevator car smoothly, is thus needed.